1. Field of the Invention
The present invention relates to a degassing apparatus for removing a gas contained in a liquid.
2. Description of Related Art
The gases dissolved in a liquid can cause various problems, such as corrosion of the pipes through which the liquid flows, a decrease in the pressure or heat exchange rate because of the formation of bubbles, and unevenness in the coating of the liquid because of the bubbles formed. For this reason, degassing is necessary, depending on the purposes and uses of the liquid. For example, sufficient degassing is desired prior to the use of a liquid for cleaning wafers or the like in a semiconductor device manufacturing process.
Degassing a liquid (a degassing target liquid) may be carried out with the use of a degassing apparatus disclosed in JP 9-57009A (see FIG. 11). In the degassing apparatus 101 shown in FIG. 11, a degassing element 103 is placed in a vacuum chamber 102, and the ends of the degassing element 103 are fixed to the vacuum chamber 102 by connecting members 104. The degassing element 103 has a structure in which joint members 106 are fitted onto the ends of a tube bundle 105, in which two or more gas-permeable tubes are bundled together. Each of the connecting members 104 and the degassing element 103 are connected by inserting an end portion of the degassing element 103 together with the joint member 106 into the interior of the connecting member 104 and screwing a fastening means 108 onto the connecting member 104 in conjunction with a fastening ferrule 107. Each of the connecting members 104 and the vacuum chamber 102 are joined by inserting the connecting member 104 into an opening of the vacuum chamber 102 and screwing a fastening means 110 onto the connecting member 104 while disposing an O-ring 109 between the inner wall of the vacuum chamber 102 and the connecting member 104. The O-ring 109 also serves to enhance the hermetic sealing of the interior of the vacuum chamber 102.
A degassing apparatus disclosed in JP 9-187602A, for example, also may be used for degassing a liquid. This degassing apparatus has a structure similar to the above-described degassing apparatus 101, in which a degassing element is placed inside a vacuum chamber, but the way in which the degassing element is fixed to the vacuum chamber is different from the degassing apparatus 101. The method of fixing the degassing element in the degassing apparatus disclosed in JP 9-187602A is described with reference to FIG. 12. A degassing apparatus 121 shown in FIG. 12 has a structure in which an end portion of a degassing element 123 is fixed to a vacuum chamber 122 by a connecting member 124. The degassing element 123 has a structure in which a tube bundle 125 including at least two gas-permeable tubes bundled together is inserted in an outer tube 126 made of a fluoropolymer and the tube bundle 125 and the outer tube 126 are hermetically heat-sealed together. The connecting member 124 and the degassing element 123 are connected by inserting an end portion of the degassing element 123 into the interior of the tubular connecting member 124 so that the end portion is abutted to a pipe 127, which has a tapered end portion, and thereafter screwing a fastening element 130 onto the connecting member 124 so that the outer tube 126 is pressed against the inner peripheral surface of the connecting member 124. The connecting member 124 and the vacuum chamber 122 are joined by screwing the connecting member 124 into the vacuum chamber 122 so as to fit a male screw portion 128 formed on the outer peripheral surface of the connecting member 124 onto a female thread portion 129 formed on the inner peripheral surface of an opening of the vacuum chamber 122.
In the degassing apparatuses disclosed in JP 9-57009A and JP 9-187602A, the connecting of the degassing element and the connecting members or the joining of the vacuum chamber and the connecting members is accomplished by a fastening structure that is effected by screwing a fastening means that engages with a connecting member.
In order to accomplish this type of fastening structure, however, various members used for the connecting and joining, such as connecting members, fastening means, joining members, and fastening ferrules, are required to have high dimensional accuracy. Therefore, the just-mentioned fastening structure has been a cause of high manufacturing cost of the degassing apparatus.
In addition, the fastening structure effected by screwing a fastening means may suffer from a decrease in the screwing force (torque) over time. Consequently, the above-described connections and joining may become deficient over time, thereby lowering the hermetic sealing of the vacuum chamber or causing leakage of the liquid from the portions where the connecting members and the degassing element are connected. Although it is possible to retain the connections and joining more reliably by increasing the screwing force of the fastening means, an excessively large screwing force may result in breakage of the connecting members and the fastening means, which means that there is a limit to the amount that the screwing force can be increased.
Furthermore, depending on the purpose of use of the degassing apparatus, such as the use in a manufacturing process of semiconductor devices, the connecting members and fastening means may be made of a resin such as a fluoropolymer, in order to prevent the liquid from being contaminated by ion component. In such cases, the decrease in the screwing force over time is more apt to occur than when those members are made of a metal, and moreover, the increasing of the screwing force is more likely to result in breakage of the connecting members and fastening means.